Ethylene/propylene copolmers and terpolymres (EPRs) currently being used for insulating medium voltage cables. These polymers are generally crosslinked using peroxides and are utilized in combination with various additives and fillers. As insulation for cables, the EPRs require a filler for reinforcement because of their lack of strength. A filler is also required for processing, e.g., in an extruder, since unfilled EPRs do not extrude smoothly.
It is desirable to add polyethylene glycol, a well known water tree growth inhibitor, to medium voltage insulation. Water trees occur when an organic polymeric insulation material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. They are called trees because they appear to have a trunk with branches. Water trees grow from points in the insulation where there are voids or contaminants such as metal, lint, or other foreign material. Since it is difficult to eliminate voids or extraneous matter from the insulation, water tree growth inhibitors have been developed. The aforementioned polyethylene glycol is one that is found to be superior in its inhibiting qualities. Unfortunately, polyethylene glycol is found to degrade EPRs when crosslinked specimens are exposed to accelerated heat aging tests at elevated temperatures.